Then there’s the possibility that play can moderate stress. Humans feel the need to blow off steam after a long week, and it seems that the proverb “all work and no play makes Jack a dull boy” may apply to animals, too—play-deprived rats become aggressive, for example. Wrestling and play fights activate the same neural pathways used in real fights, so play could train animals to deal with stressful situations. Conversely, when animals experience too much stress, play is often one of the first luxuries to go. An animal that is exhausted or starving must focus all resources on survival.
To some extent, human play comes at the expense of animals’ play. As humans urbanize animals’ natural environments, we take the land for our own purposes; what used to be animals’ playgrounds are now our playgrounds, streets, and buildings. Playing squirrels are killed by cars when they chase each other onto roads, and kangaroos have taken to holding their boxing matches on suburban thoroughfares. Many animals in disrupted habitats become more stressed as they clamor to find homes and food, which leaves less time and energy for play.
On the other hand, by posing new challenges for animals, urbanization also increases cognitive flexibility and brain size of urban animals. The demands of an urban life require ingenuity, and animals that seek out new niches in the environment are more likely to survive. If creativity is a signal of fitness, then our sledding crow friend will do just fine for himself in the city.
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